Betel nut chewing is associated with oral cancers and diabetes. This study investigated whether betel nut chewing could be associated with hypertension in Taiwanese patients with type 2 diabetes mellitus (T2DM). The data of a total of 81,226 (37,226 men and 44,000 women) patients with T2DM obtained from a cross-sectional telephone survey in a national sample of diabetic patients in Taiwan were analyzed. Hypertension was defined by a positive history or reported systolic blood pressure>or=140 mmHg and/or diastolic blood pressure>or=90 mmHg. Analyses were performed in separate sexes with consideration paid to the potential confounding effects of age, diabetic duration, body mass index and smoking. The prevalences of betel nut chewing in men and women were 20.4% and 1.1%, respectively. Betel nut chewing was more common in the younger age groups of the male sex. The multivariate-adjusted odds ratios for hypertension in chewers vs. non-chewers were 1.067 (1.007-1.131) and 1.897 (1.534-2.346) for men and women, respectively. In multiple linear regression, although no adjustment was made for the use of antihypertensive agents, betel nut chewing was significantly associated with blood pressure, with regression coefficients of 0.958+/-0.163 (SEM) for systolic and 0.441+/-0.108 for diastolic blood pressure in men; and the respective values for women were 1.805+/-0.618 and 1.198+/-0.393. In conclusion, betel nut chewing was significantly associated with hypertension in Taiwanese patients with T2DM and the association was stronger in women.